1. Field of the Invention
The present invention relates to a dry-etching apparatus for fine-etching a semiconductor device and a method of fabricating a semiconductor device by forming interconnection and the like by dry etching.
2. Description of the Related Arts
One of techniques for fine-etching a semiconductor device is a dry etching technique. In dry etching, an etching gas is introduced into a vacuum chamber, a high-frequency bias or xcexc wave is applied to the gas to generate a plasma, the plasma of the etching gas is generated, and a thin film such as a polysilicon film or an Alxe2x80x94Cuxe2x80x94Si film formed on a semiconductor wafer is etched by radicals and ions generated in the plasma. A photoresist film on which a mask pattern is transferred is formed on the thin film, and only the portion which is not covered with the photoresist film is removed by dry etching, thereby forming a semiconductor device structure in which interconnections, electrodes, and the like are integrated on the wafer.
An etching mechanism in dry etching will be briefly described by using an example of etching an Si film with chlorine gas. The chlorine gas introduced in an etching apparatus and chlorine radicals generated in the plasma are adhered on the surface of Si. Cations generated in the plasma are also incident on the surface, so that the surface is locally heated. By the heating, Si reacts with chlorine to form a reaction by-product and is desorbed. By repeating the above, etching on the Si film progresses.
In a pressure region where etching is usually performed, since the mean free path of the reaction by-product is equal to or shorter than 1 cm and is short as compared with the size (height of about 20 cm) of the etching apparatus, the reaction by-product generated on the surface of the wafer has a diffusion phenomenon by collision with other gas molecules. Consequently, the reaction by-product has the probability that it is incident on the wafer. When the incident reaction by-product is adhered, the progress of etching is disturbed. In the case where an incident flux of the reaction by-product has a distribution in the wafer surface, since etch rate decreases where the incident amount is large, it is difficult to perform uniform etching in the wafer surface.
With respect to the incident flux of the reactive by-product in the center portion and that in a peripheral portion of the wafer, although the center portion is surrounded by a reaction by-product generating portion, the peripheral portion has the reaction by-product generating portion only on one of the sides. The incident flux of the reaction by-product in the wafer center portion is less than that in the wafer peripheral portion. As a result, in an etching apparatus of a uniform ion incident amount, the etch rate in the wafer peripheral portion is higher than that in the peripheral portion. Further, the reaction by-products are adhered on side faces of a pattern. When the adhering amount is large, the pattern becomes thick. When the adhering amount is small, side-etching occurs. In order to uniform an etched shape in the wafer surface, it is therefore necessary to uniform the incident amount of the reaction by-products. To be more accurate, since a chip is not formed in the region of about 3 mm of the periphery of the wafer, it is necessary to uniform the incident amount of the reaction by-products in the region except for the periphery of 3 mm of the wafer.
In a conventional etching apparatus, the distribution of the incident flux of the reaction by-products is controlled by optimizing a gas flow, installing a focusing ring, and the like. However, as the gas pressure in the etching process is becoming lower, it is becoming difficult to improve the uniformity only by the gas flow. When a focusing ring is installed to distribute plasma density, in association with increase in wafer diameter, an excessive distribution occurs in the plasma density, a charge distribution occurs in the wafer surface, and the probability that the semiconductor device is destroyed increases. Further, as the diameter of the wafer increases, the height of the focusing ring has to be increased. When the height is increased, however, the reaction by-product is adhered to the focusing ring and becomes a cause of a foreign matter and a particle. That is, in the conventional etching apparatus, it is difficult to uniformly process a wafer having a large diameter of about 12 inches.
It is an object of the invention to provide a dry etching apparatus capable of obtaining a uniform etch rate and a uniform etched shape in a wafer surface by uniforming the number of re-incident times of a reaction by-product and a method for fabricating a semiconductor device, having such a dry etching process.
The object is achieved by uniforming the thickness of a near-surface region. Specifically, at the time of etching a wafer of a large diameter, the distance between the wafer and a member facing the wafer to a predetermined value, thereby uniforming a distribution in the surface, of the thickness of the near-surface region.
First, the incident mechanism of the reaction by-product will be described. The reaction by-product generated on the wafer moves according to a diffusion phenomenon. To be specific, since the mean free path of the product generated on the wafer is about 1/100 of the size of the etching apparatus, the product collides with gas molecules in the apparatus. By the collision, the moving direction of the reaction by-products changes and a part of the reaction by-products moves toward the wafer. Even if the movement can be maintained in the direction of moving apart from the wafer, the reaction by-products again collide with the gas molecules. As a result, the moving direction of the reaction by-products changes and the reaction by-products are incident on the wafer again and again. By such a diffusion phenomenon, the concentration of the reaction by-products becomes high near the wafer and decreases with distance from the wafer. According to the diffusion theory, the distance of the region in which the concentration of the reaction by-products from the wafer is equal to about the radius of the wafer. On the other hand, the distribution of the reaction by-products is almost uniform in a region apart from the wafer by the radius of the wafer. The concentration of the reaction by-products in the region apart from the wafer is determined substantially by residence time of the reaction by-products. The region near the wafer in which the concentration of the reaction by-products is higher than the concentration determined by the residence time will be called a near-surface region. The thickness of the near-surface region changes, as will be described hereinlater, depending on the position of the wafer, gas pressure, and gas flow rate.
As described above, by forming the near-surface region, the reaction by-products are incident on the wafer again and again. The number of colliding times of the reaction by-products with the gas molecules in the near-surface region is obtained by a mean free path (L) to the thickness (D) of the near surface region, that is, D/L. Since the probability that the reaction by-products move in the direction toward the wafer and the probability that the reaction by-products move in the direction apart from the wafer by a single collision are equal to each other, in the half of the D/L times of collisions, the reaction by-products are incident on the wafer. That is, the number of re-incident times is obtained by D/2L from the thickness (D) of the near-surface region and the mean free path (L).
In the case where the portion facing the wafer has a shower-plate structure for introducing gas, when the gas flow rate increases, the near-surface region becomes smaller, and the number of re-incident times of the reaction by-products (re-incident number of times) decreases according to a diffusion equation. When the gas pressure is increased, the diffusion coefficient of the reaction by-products becomes smaller, so that the near-surface region is narrowed. When the gas flow rate increases by 100 sccm, the thickness of the near-surface region is decreased by about 10%. When the gas pressure increases by 1 Pa, the thickness of the near-surface region is decreased by about 5%. When the gas flow rate is expressed as Q(sccm) and the gas pressure is expressed as P(Pa), the thickness Dc of the near-surface region in the center portion of the wafer is substantially obtained with respect to the radius (R) of the wafer as follows.
Dc=Rxc3x970.9(Qxe2x88x92100)/100xc3x970.95(Pxe2x88x921)
The number of re-incident times is proportional to the thickness of the near-surface region and the mean free path L is inverse proportional to the gas pressure. Consequently, the number of re-incident times (n=Dc/2L) increases with the gas pressure and starts decreasing at the high gas flow rate. The wafer does not always have a perfect circle shape. In the case where the wafer does not have a complete circle shape, the radius of the wafer is defined as the half of the longest distance from end to end of the wafer.
Since the reaction by-products are generated around the center portion of the wafer, the thickness of the near-surface region in the center portion of the wafer is about the radius of the wafer. Since no reaction by-product is generated on the outside of the wafer and the wafer peripheral portion serves as an exhaust port, the thickness of the near-surface region in the wafer peripheral portion is about the half of that in the center portion, that is, about the half of the radius of the wafer. FIG. 1 shows the relation between the distance from the center portion of the wafer and the near surface region in the case of etching Al on a 12-inch wafer (having a diameter of 300 mm) with a gas pressure of 1 Pa and a flow rate of Cl2+BCl3 gas of 300 sccm. When the wafer facing surface is sufficiently apart from the wafer, the near-surface region extends over the wafer as shown by 101 in FIG. 1. The number of re-incident times of reaction by-products is proportional to the near-surface region. In a region where the near-surface region in the wafer center portion is large, the number of re-incident times increases, and the reaction by-products are adhered to a matter to be etched, so that the etch rate becomes low. On the other hand, the near-surface region is small in the peripheral portion of the wafer, the number of re-incident times is small, and the etch rate is high.
In practice, there is a wafer table (processing stand) on which a wafer is placed. When the wafer table is also taken into account, a susceptor exists on the outside of the wafer peripheral portion in order to place the wafer in a fixed position. Consequently, the reaction by-products are reflected by the surface of the susceptor, and a part of the reflected reaction by-products is again incident on the peripheral portion of the wafer. When the radius of the wafer table is larger than the wafer radius by 20 mm, that is, there is a susceptor having a width of 20 mm around the wafer, as shown by the curve 101 in FIG. 1, the near-surface region in the wafer periphery is about ⅔ as compared with that in the center of the wafer. The curve 101 in FIG. 1 showing the distribution in the wafer surface, of the thickness of the near-surface region in the case of 12 inches and the gap of 140 mm or larger corresponds to a curve 201 indicative of the distribution in the wafer surface, of the number of re-incident times in FIG. 2. In this case, as shown by the curve 201 in FIG. 2, the number of re-incident times in the wafer peripheral portion is about ⅔ of that in the wafer center portion. In the near-surface region in the wafer peripheral portion, the effect of returning the reaction by-products decreases with the distance. Consequently, when the width of the susceptor is 170 mm or less, the thickness of the near-surface region is almost proportional to the square root of the width of the susceptor. When the width of the susceptor is 170 mm or wider, the thickness of the near-surface region in the peripheral portion does not change. When the width of the susceptor is 170 mm or less and is expressed as (d), the thickness De of the near-surface region in the wafer peripheral portion is expressed by De=Dcxc3x97(0.5+{square root over ( )}d). When k denotes a proportionality constant and the unit of (d) is cm, k=0.12.
As described above, since the thickness of the near-surface region is distributed above the wafer, the number of re-incident times of the reaction by-products above the wafer in the wafer peripheral portion is smaller than that in the wafer center portion (curve 101 in FIG. 1). As a result, even if the plasma is uniform, the incident flux of the reaction by-products is distributed. It is therefore difficult for the conventional apparatus to obtain an uniform etched shape at a uniform etch rate.
Since the cause of nonuniformity is nonuniform thickness of the near-surface region above the wafer in such a manner that the near-surface region is thin in the wafer peripheral portion and is thick in the center, by uniforming the near-surface region, the uniformity in the surface can be improved.
When a free space extends over the wafer, the near-surface region can extend to a distance equal to about the radius of the wafer. However, in the case of an apparatus structure in which the distance (gap) between the wafer and the wafer facing surface is shorter than the radius of the wafer, the near-surface region in the wafer center portion is disturbed by the wafer facing surface. When the wafer facing surface is rough and the roughness is small, the gap is between the wafer surface and the portion closest to the wafer surface. When the wafer facing surface is concave or convex, the gap varies according to positions in the wafer. The gap at a position in the wafer is the distance between the wafer and the wafer facing surface on the perpendicular line in the center of the wafer. Unless otherwise specified, the gap will be explained as a gap in the wafer center portion.
Specifically, a 12-inch wafer is used and the thickness of the near-surface region with the gap of 110 mm and that with the gap of 80 mm are indicated by curves 102 and 103 in FIG. 1, respectively. Since the thickness of the near-surface region De in the wafer peripheral portion is about 80 mm, the gap is set to 80 mm and the thickness of the near-surface region in the wafer center portion is adjusted to that in the peripheral portion, thereby uniforming the thickness of the near-surface region above the wafer surface. The curve 102 indicative of the distribution in the wafer surface of the thickness of the near-surface region when the gap is 110 mm in FIG. 1 corresponds to a curve 202 in FIG. 2. The curve 103 indicative of the distribution in the wafer surface of the thickness of the near-surface region when the gap is 80 mm in FIG. 1 corresponds to a curve 203 in FIG. 2. As obviously understood from the comparison between FIGS. 1 and 2, the number of re-incident times of the reaction by-products is proportional to the thickness of the near-surface region. When the plasma is uniform, it is therefore sufficient to adjust the gap so as to be equal to the near-surface region De in the wafer peripheral portion. A desired gap G is formalized as follows by including the gas pressure P, the gas flow rate Q and the wafer radius R on the basis of the equation of the thickness of the near-surface region.
G=Rxc3x970.9(Qxe2x88x92100)/100xc3x970.95(Pxe2x88x921)xc3x97(0.5+k{square root over ( )}d)
As for an effect on improvement in uniformity, when the gap is shorter than the thickness of the near-surface region in the wafer center portion, the gap G0 is expressed as follows.
G0=Rxc3x970.9(Qxe2x88x92100)/100xc3x970.95(Pxe2x88x921)
That is, the uniformity is improved with the gap equal to or shorter than G0, and uniformity is achieved when the gap is about G. When the gap is further shortened, nonuniformity may occur again due to occurrence of pressure variations and an unstable plasma. It is therefore desirable to control the uniformity in the range from G to G0.
FIGS. 3, 4, 5, and 6 show the gap dependency of the distribution in the wafer surface of the number of re-incident times with respect to the wafer diameter, gas pressure, gas flow rate, and susceptor width.
FIG. 3 shows the relation between the distribution of the number of re-incident times and the distance between a processing stage and a surface facing the processing stage in the case of performing a plasma process with the gas pressure of 1 Pa and the Cl2+BCl3 gas flow rate of 100 sccm. A curve 301 in FIG. 3 is a curve indicative of a distribution in the wafer surface of the number of re-incident times in a 6-inch wafer, a curve 302 is a curve indicative of a distribution in the wafer surface of the number of re-incident times in an 8-inch wafer, a curve 303 is a curve expressing a distribution in the wafer surface of the number of re-incident times in a 12-inch wafer, and a curve 304 is a curve expressing a distribution in the wafer surface of the number of re-incident times in a 16-inch wafer. Since the near-surface region in the wafer peripheral portion is as short as about ⅔ of that in the center portion, as understood from the curve 302 in FIG. 3, in order to make the thickness of the near-surface region in the wafer center portion of the 8-inch wafer almost equal to that in the wafer peripheral portion, it is sufficient to adjust the distance (gap) between the wafer and the wafer facing surface to about 70 mm which is ⅔ of the wafer radius. When the gap is 70 mm, the number of re-incident times is almost uniform. On the other hand, with respect to a 6-inch wafer with the same flow rate, it is sufficient to adjust the gap to about 50 mm as shown by the curve 301. In the case of the 12-inch wafer (curve 303), it is sufficient to adjust the gap to about 100 mm. In the case of a 16-inch wafer, it can be estimated that the number of re-incident times becomes almost uniform when the gap is 130 mm. It is not always necessary to set the distribution of the number of re-incident times to zero. When the distribution is set to 10% or lower, preferably, 5% or lower, the effect on uniformity is improved.
When the gap is narrowed further, although the near-surface region in the wafer peripheral portion is also hindered by the wafer facing surface, the thickness of the near-surface region in the center portion and that in the peripheral portion become almost equal to each other. Consequently, even when the distance between the wafer and the wafer facing surface is set to ⅔ of the radius of the wafer or narrower, the uniformity of the number of re-incident times of the reaction by-products in the wafer surface is improved within the range where the gas pressure in the wafer surface can be held uniform. As described above, by setting the distance between the wafer and the wafer facing surface to ⅔ of the wafer radius or narrower, the incident flux of the reaction by-product becomes uniform, so that the etch rate and the etched shape become uniform in the wafer surface.
However, in the case of using, especially, a parallel plate etching apparatus, when the gap is extremely narrowed to 50 mm or less, the gas pressure distribution in the wafer surface becomes large. Particularly, in the case of a low gas pressure of a few Pa or a high gas flow rate of few hundreds sccm, the gas pressure distribution becomes large and it becomes difficult to assure the etch rate and the uniformity. It is therefore preferable not to extremely narrow the gap.
It is understood from FIG. 4 of the distribution of the number of re-incident times with respect to gas pressure that the gap has to be narrowed as the gas pressure increases. FIG. 4 shows a case where the wafer diameter is 12 inches (300 mm) and the gas flow rate is 300 sccm. In FIG. 4, a curve 401 is a curve indicative of a distribution in the wafer surface of the number of re-incident times with the gas pressure of 0.2 Pa, a curve 402 is a curve indicative of a distribution in the wafer surface of the number of re-incident times with the gas pressure of 1 Pa, a curve 403 is a curve indicative of a distribution in the wafer surface of the number of re-incident times with the gas pressure of 3 Pa, and a curve 404 is a curve indicative of a distribution in the wafer surface of the number of re-incident times with the gas pressure of 5 Pa. When the gas pressure is 1 Pa or lower (curves 401 and 402), the number of re-incident times of the reaction by-products becomes uniform (distribution is almost 0%) with the gap of 80 mm. When the gas pressure is 5 Pa (curve 404), it is necessary to set the gap to about 60 mm. However, when the gap is 60 mm, a gas pressure distribution may occur. Consequently, a low gap pressure is preferable.
FIG. 5 is a diagram showing the gas pressure dependency of the distribution of the number of re-incident times. In FIG. 5, a curve 501 is a curve indicative of a distribution in the wafer surface of the number of re-incident times with a gas flow rate of 100 sccm, a curve 502 is a curve indicative of a distribution in the wafer surface of the number of re-incident times with a gas flow rate of 300 sccm, and a curve 503 is a curve indicative of a wafer in-surface distribution of the re-incident number of times with a gas flow rate of 500 sccm. At the gas flow rate of 500 sccm (curve 503), when the gap is set to 60 mm, the number of re-incident times is not distributed and is uniform.
FIG. 6 shows dependency on the susceptor width of the distribution in the wafer surface of the number of re-incident times. In FIG. 6, a curve 601 is a curve indicative of a distribution in the wafer surface of the number of re-incident times when there is no susceptor. A curve 602 is a curve indicative of a distribution in the wafer surface when the width of the susceptor is 20 mm. A curve 603 is a curve indicative of a distribution in the wafer surface when the width of the susceptor is 50 mm. A curve 604 is a curve indicative of a distribution in the wafer surface when the width of the susceptor is 100 mm. When the width of the susceptor is 20 mm (curve 602), the gap with which the number of re-incident times becomes uniform is about 80 mm. When the width is increased to 50 mm and 100 mm, the necessary gap is widened to 90 mm and 100 mm, respectively.
In the case of the 8-inch wafer, in order to assure uniformity, it is sufficient to set the gap to about 70 mm. In the conventional apparatus structure, however, in some cases, it is difficult to stably generate a high-density plasma. For example, in the parallel plate type etching apparatus, it is difficult to generate the high-density plasma with the gap of 300 mm or wider, and the etch rate becomes extremely low when the gap is 70 mm. In order to compensate the reduction in density, since the reduction in density is caused by decrease in electric field due to diffusion of the plasma and increase in gap, it is necessary to take measures such as the apparatus structure capable of confining the plasma, suppression of the plasma diffusion by application of a magnetic field and increase in high frequency power.
In an apparatus of an inductive coupled type, the plasma generating portion is on the wall side of the processing chamber in the peripheral. Consequently, when the gap is narrowed, the density is lowered in the wafer center portion of, especially, a wafer of a large diameter, and it becomes difficult to generate a uniform plasma. In the case of the etching apparatus of the inductive coupled type, it is therefore necessary to have a structure such that a high-frequency introducing antenna is disposed close to the center portion.
In the case of a magnetic UHF etching apparatus, micro-waves are allowed to propagate into an etching chamber and plasma is generated by electron cyclotron resonance (ECR) of the micro-waves and a magnetic field generated by a solenoid coil provided around the apparatus. Consequently, by controlling the ECR point by the magnetic field, theoretically, high-density plasma can be generated in an arbitrary position on the wafer. When the wafer is disposed close to a micro-wave window positioning in the wafer facing portion, however, since the apparatus itself is a cavity for the micro-waves, resonance cannot be easily obtained and plasma becomes unstable. This can be dealt with by adjusting the height of the cavity in the upper part of the micro-wave window. It is necessary to adjust the height of the cavity of typically about 70 mm to about 150 mm.
On the other hand, in the case of introducing the micro-waves by the antenna in the micro-wave etching apparatus, since the antenna is a resonator, the gap can be freely adjusted. With respect to the increase in wafer diameter, in the case of micro-waves having a wavelength of about 120 mm, there is the possibility that uniformity cannot be assured due to occurrence of a node. On the other hand, in the case of a UHF wave having a wavelength of about 600 mm, there is no such a problem. When high frequencies are introduced by an antenna, however, there is a case that an amount of incident ions increases in the peripheral portion by the electric field in the antenna peripheral portion. It is therefore desirable to set the gap to about 50 mm or wider.
The means for solving the problem in the etching apparatus on the assumption that the apparatus has the shower plate for introducing gas having the wafer facing surface which is almost flat and the susceptor has a height almost equal to that of the wafer and has a width of about 20 mm has been described above. As means for uniforming the near-surface region above the wafer, there are a structure (V-shape or recessed shape) in which the distance between the center portion of the wafer facing surface and the wafer is longer than that in the wafer peripheral portion, a method of widening the susceptor in the wafer peripheral portion and uniforming the near-surface region by a gap wider than ⅔ of the radius of the wafer, and a method of providing a ring (focusing ring) which is high so as to surround the wafer in the wafer peripheral portion and uniforming the near-surface region by a gap wider than ⅔ of the radius of the wafer.
Other than the above, there is a method of setting the positions of holes of gas feed of the shower plate in the wafer center portion having a diameter of about ⅔ to ⅓ of the wafer diameter, thereby pushing the reaction by-products in the center portion by the flow of the gas and uniforming the near-surface region even with a gap which is wider than ⅔ of the radius of the wafer.
Although attention has been paid above to the number of re-incident times of the reaction by-products as a factor of determining the distribution of the etch rate, the etch rate also depends on an ion current. The dependency depends on a film to be etched. Although the etch rate of the Al film hardly changes within the range of an ion current (2 to 4 mA/cm2) with which etching is usually performed, in the case of a polysilicon film or an oxide film, the etch rate is almost proportional to an ion current density. With respect to the polysilicon or oxide film etching, consequently, there is a method of uniforming the etch rate in the wafer surface by setting the distribution in the wafer surface of the ion current density so as to open downward in accordance with the distribution of the wafer surface of the number of re-incident times of the reaction by-product.
The control of the distribution of the ion current density differs according to plasma generating means. Specifically, in the case of a magnetic micro-wave etching apparatus or a magnetic UHF etching apparatus each having two or more solenoid coils, by controlling the magnetic field profile, the distribution in the wafer surface of the ion current density is controlled. When the magnetic field profile is set to open upward, the wafer peripheral portion is away from the plasma, so that a distribution which opens downwards is obtained. In the case of an apparatus of an inductive coupled type having a coil antenna of two or more turns, the diameter of the first antenna and that of the second antenna are different from each other. The distribution of the ion current density is controlled by means for applying different currents to the first and second turns or the like. When the current for the antenna having the smaller diameter increases, the curve of the ion current density opens downwards.
When a prior art search was made after completion of the above-described invention, Japanese Unexamined Patent Application No. Hei-9-134906 as a known technique was found. The publication of the known technique describes an apparatus for etching a wafer having a diameter of 200 mm in such a manner that the distance between an upper electrode and a susceptor for holding a wafer, which faces the upper electrode is 70 mm and an SiO2 film is selectively etched for an SiN film. The publication, however, does not suggest the relation between the diameter of the wafer and the gap as described in the present invention.
With the construction, by uniforming the incident flux of the reaction by-product in the wafer surface, uniform etching in the wafer surface can be achieved.
Further, according to the present invention, the processing accuracy of a pattern in the wafer surface is improved, that is, variations in shape in the surface are reduced. Consequently, variations in wiring resistance and capacitance of a semiconductor device and variations in gate length are eliminated. Thus, a semiconductor device having little variations in device characteristics can be easily mass-produced.